1. Field of the Invention
The field of the invention is related to quantitative fit tests (QNFT) used to grade respiratory protective devices. More particularly, the invention is related to a novel and non-obvious quantitative fit test for protective respiratory devices that would be used in the case of chemical, biological, radiological and nuclear (CBRN) hazards.
2. Description of the Related Art
Respiratory protection devices used for military and homeland defense applications must protect against a wide range of chemical, biological, radiological and nuclear (CBRN) hazards. The effectiveness of a CBRN respirator system to protect the wearer against airborne hazards relies on both the performance of the respirator filtration system and the respirator-wearer seal. A properly fitted and sealed respirator will form a tight impenetrable bond at the respirator and wearer interface, while an improperly sealed respirator will allow hazardous materials to circumvent the filtration system and enter the respirator. The effectiveness of a respirator to seal off the contaminated area to the wearer and protect against airborne hazards is quantified in terms of a fit factor (FF). The FF, which is a quantitative estimate of a respirator fit, is defined as the ratio of the challenge concentration outside the respirator to the concentration measured inside the respirator facepiece.
The quantitative fit test (QNFT) provides one with what many consider to be the most accurate, convenient, and non-subjective form of testing. The test results are immediate, unambiguous, and take no more time to perform than qualitative testing methods.
Occupational Safety and Health Act (OSHA) regulations require that all employees using respirators be fit tested either annually or semi-annually, based on the hazard to which they are exposed. All qualitative fit tests are conducted in accordance with 29 C.F.R. §1910.134. The standard photometer-based QNFT method used by the U.S. military and the National Institute for Occupational Safety and Health (NIOSH) to qualify the protection level of CBRN respirators can not sufficiently quantify the FF required for biological agents. The current QNFT method uses a polydisperse corn oil aerosol challenge with a mass median aerodynamic diameter (MMAD) of 0.4 to 0.6 micrometers (μm) that is intended to represent both gas/vapor and aerosol chemical threat agents with respect to respirator (mask) seal leakage. An aerosol photometer is used to measure the relative concentration of the challenge and respirator in-mask atmosphere, which is determined by light scattering of the particles in the sample stream. The higher the fit factor, the better the mask guards against leakage. It is known that factors up to 100,000 can be measured using this method of testing.
FIG. 8 shows one popular testing method using an exposure chamber 100 to confine the generated corn oil challenge around the person 110 being fit tested. After donning the respirator 120 and entering the exposure chamber 100, the person 110 performs a series of exercises designed to stress the face seal of the respirator to determine whether the face seal performs satisfactorily under actual use in a potentially contaminated area. The respirator 120 must be equipped with High Efficiency Particulate Air (HEPA) filters (HEPA filters having been tested to assure removal of 99.97% of particles 0.3 μm in size) that prevent the aerosol challenge from penetrating. Thus, when a fit test is performed, it is assumed that all particles sampled from inside the respirator have entered through a face seal leak.
A popular device for conducting a QNFT test in the industry is known as a PORTACOUNT® (available from TSI, St. Paul, Minn.). The U.S. military will sometimes fit test respirators for use in the workplace with the PORTACOUNT because of its ease of use and simplicity. The PORTACOUNT® is a portable particle-counting instrument that uses condensation particle counting technology to measure the number concentration of particles both outside and inside the respirator to determine the FF number. The instrument utilizes particles found in the ambient air (the majority of particles typically occur in the 0.01 to 0.1 μm range) as the test challenge. This instrument also eliminates the need for aerosol generators and test chambers. The PORTACOUNT® is capable of measuring FF values of up to 10,000 or higher depending on the ambient particle background concentration.
Although the above QNFT methods may effectively qualify the protection afforded against toxic chemical gas/vapor and particulate hazards, these methods do not provide an effective measurement of protection against biological agents. Biological weapons pose a unique threat to military and civilian populations since they are usually invisible, odorless, exhibit latent effects, and are not easily detectable compared to conventional chemical warfare agents. Infectious biological agents such as anthrax, small pox, and tularemia are of particular concern since inhalation of a relatively small number of organisms can result in a lethal dose. Furthermore, biological aerosols (bio-aerosols) are more likely to be present on ambient particulate matter or exist as conglomerates (i.e., particles consisting of multiple organisms) that range from 1 to 5 μm in diameter.
Neither the photometer nor the PORTACOUNT® QNFT devices have the ability to determine the size of the particulate challenge. Furthermore, corn oil and ambient aerosol QNFT challenges as currently used in these methods are not good simulants of biological agents. Both test challenges exist as polydisperse aerosols consisting of mostly smaller particles and relatively few particles similar in size to the vast majority of bio-aerosol threat agents (i.e., >1 μm). Thus, the respirator is challenged with a low concentration of particles comparable in size to biological agents. As previously mentioned, the photometer-based QNFT method provides a FF that is based on the relative concentration of particles penetrating the respirator seal. The FF is determined directly from the voltage reading from the light-scattering photometer aerosol sensor and is therefore not an absolute measurement of concentration. The photometer can be calibrated to yield a total mass concentration measurement (e.g., mg/m3), but this is not typically done for quantitative fit testing applications. Toxicological effects of chemical agents are a function of the mass concentration (effective dose). For biological agents, however, it is the number of viable organisms inhaled and not the mass concentration that determines the risk of a life-threatening exposure. With no size-specific count measurement capability, the true number of simulated biological particles penetrating the seal cannot be determined using the conventional photometer or particle-counting QNFT methods.
Another shortcoming of conventional QNFT methods is that they lack sufficient sensitivity to measure FF values required for highly lethal biological agents. A relatively small number of these hazardous organisms can cause severe health effects when inhaled. Hence, the level of respiratory protection required for biological agents is in general at least an order of magnitude higher than that needed for chemical agents. Furthermore, background particles generated by the mask wearer during fit testing (typically from exhaled breath) can result in artificially low FF values when particle-counting QNFT instruments are used. In order to measure the FF required for biological protection and overcome measurement bias caused by background particles, the challenge concentrations of simulated biological particles needs to be several orders of magnitude higher than is obtainable using conventional QNFT methods.
Therefore, there is a need in the art to provide a system and method of QNFT testing that provides a way to create a challenge atmosphere of particles that are comparable in size to bio-hazardous agents, and a way to count these particles according to size to fit test the mask and/or respirator system and determine its effectiveness against bio-hazardous agents.